Disc drives are used for data storage in modern electronic products ranging from digital cameras to computer systems and networks. A typical disc drive includes a head-disc assembly (HDA) housing the mechanical portion of the drive, and a printed circuit board assembly (PCBA), attached to the head-disc assembly. The printed circuit board assembly controls operations of the head-disc assembly and provides a communication link between the head-disc assembly and a host device served by the disc drive.
Typically, the head-disc assembly has a disc with a recording surface rotated at a constant speed by a spindle motor assembly and a head stack assembly positionably controlled by a closed loop servo system. The head stack assembly supports a read/write head that writes data to and reads data from the recording surface. Disc drives using a magneto resistive read/write head typically use an inductive element, or writer, to write data to the information tracks and a magnetoresistive element, or reader, to read data from the information tracks during drive operations.
One type of data recorded to and read from the information tracks is servo data. Servo data, including a physical track identification portion (also referred to as a servo track number or physical track number), written to the recording surface define each specific physical track of a number of physical tracks written on the recording surface. A servo track writer is traditionally used in writing a predetermined number of servo tracks to each recording surface during the manufacturing process. The servo tracks are used by the closed loop servo system for controlling the position of the read/write head relative to the recording surface during disc drive operations.
High performance disc drives achieve areal bit densities in the range of several gigabits per square centimeter (Gbits/cm2). Higher recording densities can be achieved by increasing the number of bits per centimeter stored along each information track, and/or by increasing the number of tracks per centimeter written across each recording surface. Capacity increases gained through increasing the bits per centimeter stored on each track generally requires improvements in the read/write channel electronics to enable data to be written to and subsequently read from the recording surface at a correspondingly higher frequency. Capacity increases gained by increasing the number of tracks per centimeter on each recording surface generally require improvements in servo control systems, which enable the read/write head to be more precisely positioned relative to the information tracks.
Signal loss in reading data from an information track in a disc drive is directly proportional to the distance the read/write head is from the information track and the wavelength of the signal. Often, a reduction in an operating fly height of the read/write head accompanies density increases through increased bits per centimeter along the information track. The term operating fly height means a distance, determined for a disc drive by design, the read/write head is spaced from the disc surface to achieve an optimum read/write performance for the head media combination within a disc drive of a particular design. An abrupt reduction in the operating fly height, that may occur, for example, by encountering an asperity event, for disc drives using a magnetoresistor in the reader portion of the read/write head increases the difficulty in recovering data.
An effect of an asperity event on the magnetoresistor and the resulting response of the read channel are well known. An asperity event is characterized by a sudden increase in amplitude of the read signal relative to a normal signal baseline, followed by a decaying portion that lasts for about two microseconds. The portion of the readback signal corresponding to the thermal asperity will generally be sufficiently distorted to prevent proper decoding by the read circuit.
The read circuit and the servo circuit for both training and decoding purposes rely on a normal signal baseline. Excursions of the read signal from the normal signal baseline trigger a response by the read/write channel electronics to decrease the gain of the automatic gain control portion of the read channel, causing an inability of the read circuit and the servo circuit to properly decode the readback signal. Accordingly, monitoring the amplitude of the read signal for sudden increases in amplitude beyond the normal signal baseline followed by a data error provides a convenient means for detecting an asperity event. For reference, additional discussions concerning detection of an asperity event are provided in U.S. Pat. No. 5,995,313 issued Nov. 30, 1999 to Dakroub and U.S. Pat. No. 6,043,946 issued Mar. 28, 2000 to Genheimer et al, both assigned to the assignee of the present invention.
Data errors also can come about due to a variety of temperature, voltage, and other margin conditions present during a write operation that weaken the state of the recorded data. During a read operation those same conditions may have the effect of weakening the ability of the reader to read previously recorded data. In either case, data errors reduce the ability of the disc drive to reliably read previously recorded data.
With reduced fly heights of the read/write head that typically accompany increased liner bit density, debris generated during disc drive operations that settles on the recording surface, when encountered, presents wear problems for the active elements of the read/write head. Wear problems also can occur during extended periods of data transfer inactivity while the spindle motor is spinning by impacting debris and by windage friction. An erosion or wear of the active elements of the read/write head reduces the ability of the disc drive to reliably read and write data to the recording surface, which is a reduction in the reliability of the disc drive.
Therefore, challenges remain and needs persist for means of reducing wear of the active elements of the read/write head to improve disc drive reliability. It is to this and other features and advantages set forth herein that embodiments of the present invention are directed.